Abstract: The radius of a star is one of the most basic outputs of a stellar model. Yet
observations of low-mass eclipsing binaries show that standard evolutionary
calculations significantly underpredict their radii. This "radius
inflation" is hypothesised to be due to magnetic activity, either
inhibiting convection or blocking flux at the surface through starspots. We
will test these ideas, using WIYN Hydra to estimate the average radii of {it
single} magnetically active, low-mass stars in the young Pleiades cluster. Our
technique models the projected radii determined from the product of
spectroscopic measurements of rotational broadening and published rotation
periods to yield average radii for groups of stars. Large, homogeneous
statistical samples are required and owing to its proximity and richness, the
Pleiades is the ideal target. In particular, it allows sensitive average radius
estimates in several mass bins that straddle the fully convective boundary,
where opposite trends with mass are predicted by the two competing flavors of
magnetic radius inflation. Although our primary interest is stellar structure,
there are significant implications for exoplanet science since improved stellar
models and radii, particularly for low-mass and pre-main sequence stars, will
lead to more accurate age estimates for stars and their planetary systems and
better radii and densities for their transiting exoplanets.